In semiconductor device production, circuit patterns are commonly written onto semiconductor substrates by lithographic processes. In image-projection lithography, radiation is projected through a master mask or reticle onto a photoresist layer on the substrate in order to create the circuit patterns. In direct-write systems, on the other hand, the circuit pattern is written onto the substrate by directly modulating a beam of light or electrons, which is then incident on the photoresist layer. Masks and reticles may also be produced by this sort of direct-write process.
One method known in the art for direct writing of circuit patterns is to scan a focused laser beam or electron beam over the surface of the substrate in a pattern of multiple, parallel lines, known as a raster. Each line consists of a single row of pixels in the master image that is to be written on the substrate. At each pixel location, the beam intensity is controlled to give the desired pixel exposure. A direct-writing photolithography system of this sort is described, for example, in U.S. Pat. No. 5,635,976, whose disclosure is incorporated herein by reference.
Alternatively, it is possible to write an entire frame of pixels on the substrate simultaneously, using a spatial light modulator (SLM) to create the desired pattern. For example, U.S. Pat. No. 5,691,541, whose disclosure is incorporated herein by reference, describes a lithography system in which a programmable array of binary light valves or switches is programmed to replicate a portion of the circuit pattern each time an illuminating light source is flashed. The substrate is mounted on a scanning stage. The stage motion and the pattern of the programmable array are synchronized with the illumination system so that each flash accurately positions the image of the pattern on the substrate. In this manner, the entire image is built up of multiple flashes. In one embodiment, the light pattern projected by the programmable array is incident not on the substrate, but rather on an electron-emitting photocathode. The light pattern causes the photocathode to emit electrons, generating a corresponding electron image that is focused onto the substrate by electron optics. By using electrons rather than photons to create the final image, it is possible to achieve higher resolution.
Another approach to electron beam shaping is described in U.S. Pat. No. 6,014,200, whose disclosure is also incorporated herein by reference. This patent describes an electron beam lithography system that uses multi-aperture arrays to shape an electron beam. The electron beam is divided up into multiple beamlets. Deflection logic is then used to blank selected beamlets in order to create selected beam patterns. The unblanked beamlets are directed onto a surface to be exposed. A moving objective lens (MOL) may be used in scanning the beam over the surface.
Other systems for direct-light lithography based on spatial light modulators are described in U.S. Pat. No. 6,285,488, U.S. Pat. No. 6,312,134, U.S. Pat. No. 6,399,261, U.S. Pat. No. 6,493,867 and U.S. Patent Application Publication 2002/0024714, whose disclosures are incorporated herein by reference, as well as in the above-mentioned U.S. Patent Application Publication 2003/0122091. In some of these systems, the SLM is operated to project gray-scale images onto the substrate, with multiple gray levels, rather than binary images as in the system described in U.S. Pat. No. 5,691,541. For example, U.S. Patent Application Publication 2002/0024714 describes apparatus for creating a pattern on a workpiece, wherein the modulating elements of the SLM can be set by drive signals to more than two different states, thus giving intermediate exposure values of the light incident on the workpiece. U.S. Pat. No. 6,285,488 describes a method for creating a large pattern on a workpiece by stitching together partial images created by different light pulses. The partial images are made to overlap one another, and are projected with reduced light intensity in the overlap region in order to reduce the visibility of the edges between the partial images.